avformat/mpegts: initialize max_packet_size when sub-demuxer

bca30570d2 added a user option to set max_packet_size replacing
a hardcoded value. This had a side-effect of leaving the field
set to 0 when packet demuxing is carried out from another demuxer
using avpriv functions, which could lead to demux failure.

Hardcoded max_packet_size inside avpriv_mpegts_parse_open to
2048000 to avoid this. Value chosen to be 10x that of default value
to accommodate large payloads.

libavformat/mpegts.c

@@ -3377,6 +3377,7 @@ MpegTSContext *avpriv_mpegts_parse_open(AVFormatContext *s)
         return NULL;
     /* no stream case, currently used by RTP */
     ts->raw_packet_size = TS_PACKET_SIZE;
+    ts->max_packet_size = 2048000;
     ts->stream = s;
     ts->auto_guess = 1;